Does flight ability differ among male morphs of the Japanese horned beetle Trypoxylus dichotomus septentrionalis (Coleoptera Scarabaeidae)?

Abstract

The Japanese horned beetle, Trypoxylus dichotomus septentrionalis, exhibits inter- and intrasexual dimorphism. Among Japanese horned beetles showing intrasexual dimorphism, minor males with shorter rudimentary horns adopt non-competitive tactics rather than competitive fighting tactics. It has been generally considered that minor males invest more in traits associated with aerial and terrestrial locomotion, such as wing characteristics, than in traits associated with fighting ability, such as horn characteristics. However, distinct alternative behaviours corresponding to alternative male phenotypes have never been detected in previous studies on T. dichotomus septentrionalis. During flight the beetles are frequently preyed upon by the Ural owl; therefore, I conducted an experiment to examine the presence of inter-morph differences in flight speed, which appears to directly affect the ability of the beetles to avoid predation. However, there was no significant difference between the flight speeds of major and minor males. Larger males were found to fly a little faster than smaller males irrespective of the traits used as a measure of body size (i.e. horn length, prothorax width, body mass, or wing loading). Thus, from the viewpoint of the flight speed, it is likely that alternative behaviour of minor males, for gaining relatively higher fitness than middle-sized males, was not seen in this species in the present study. Hence the results of the present study support the hypothesis that dimorphic allometry exists because of a continuous reaction norm that meets a ceiling, rather than developmental reprogramming at a body size threshold.